1. Technical Field
The present invention relates to a display device equipped with an optical sensor which is formed by a TFT (Thin Film Transistor) for detecting external light and an optical detector which has a capacitor connected between a pair of electrodes of the optical sensor. More specifically, the present invention relates to a display device equipped with an optical sensor and an optical detector capable of allowing a display screen to be quickly seen by quickly detecting external light particularly when a peripheral environment changes from a dark state to a bright state.
2. Related Art
As display devices, there are known various display devices such as a CRT (Cathode Ray Tube), a liquid crystal display device, an LED (Light Emitting Diode) display device, a plasma display device, and an organic EL display device. Among the various display devices, the liquid crystal display device is more widely used than the CRT for the purpose of a display in many electronic apparatuses due to the light weight, the thin thickness, and the low power consumption thereof. The liquid crystal display device displays an image thereon in such a manner that the directions of liquid crystal molecules aligned in a predetermined direction are changed by an electric field so as to change a light transmission amount of a liquid crystal layer. As the types of the liquid crystal display device, there are known a reflection type, a transmission type, and a semi-transmission type. Specifically, the reflection type has a structure in which external light transmits through a liquid crystal layer, is reflected by a reflection member, transmits through the liquid crystal layer again, and then is emitted to the outside. The transmission type has a structure in which light incident from a backlight unit transmits through the liquid crystal layer. The semi-transmission type has both characteristics of the reflection type and the transmission type.
The liquid crystal display device of the reflection type is advantageous in that the power consumption is small since the external light is used as a light source, but is disadvantageous in that an image displayed thereon is difficult to be seen in a dark place. The liquid crystal display device of the transmission type is advantageous in that an image displayed thereon is easily seen even in a dark place, but is disadvantageous in that the power consumption is large since the backlight unit is required to be turned on all the time.
In the liquid crystal display device of the semi-transmission type, one sub-pixel region has a transmission region and a reflection region. In a dark place, the backlight unit is turned on to display an image via the transmission region. In a bright place, the external light in the reflection region is used to display an image without using the backlight unit. For this reason, the liquid crystal display device of the semi-transmission type is advantageous in that the power consumption is remarkably reduced since the backlight unit is not required to be turned on all the time. Particularly, the liquid crystal display device of the semi-transmission type is widely used in portable electronic apparatuses.
Meanwhile, in the liquid crystal display devices of the transmission type and the semi-transmission type, JP-A-2007-316243 discloses a technology in which an optical sensor used to detect external light is provided in the liquid crystal display device so as to decrease brightness of a backlight unit in the case of dark external light and to increase the brightness of the backlight unit in the case of the bright external light, thereby allowing the image to be easily seen even when the peripheral brightness changes. Similarly, in the liquid crystal display device of the semi-transmission type, JP-A-2008-83313 discloses a technology in which a backlight unit is turned off so as to display an image in a reflection region in a bright place and the backlight unit is turned on so as to display the image in a transmission region in a dark place. Even in other display devices, similarly, the brightness of the display device is automatically changed so as to allow the image to be easily seen in the case where the peripheral brightness changes.
In the liquid crystal display device disclosed in JP-A-2007-316243, a photo diode provided in a liquid crystal panel is used as the optical sensor. However, when the photo diode is used as the optical sensor, a problem arises in that the number of manufacture processes of the liquid display panel increases. For this reason, in the liquid crystal display devices disclosed in JP-A-2008-83313 and JP-A-2007-279100, an optical sensor formed by a TFT simultaneously formed with a TFT for driving the liquid crystal display panel is used as the optical sensor. The optical sensor formed by the TFT functions as an optical conductive element in which a leakage current caused by optical leakage increases in accordance with an increase of illumination, where the illumination is measured in accordance with an amount in which a voltage caused by electric charge accumulated in a capacitor (condenser) decreases due to the leakage current of the optical leakage.
As described above, in the known method of measuring the illumination of the external light by using the optical sensor formed by the TFT, there are known a method in which the illumination of the external light is measured on the basis of the voltage of the capacitor after a predetermined time and a method in which the illumination of the external light is measured by measuring a time until a voltage of the capacitor becomes a value not more than a threshold value. Among the methods, an operation of a known optical sensor controller for measuring the illumination of the external light by measuring the time until the voltage of the capacitor becomes the value not more than the threshold value will be described with reference to FIG. 11.
In addition, FIG. 11 is a time chart showing waveforms of the respective parts when the known optical sensor controller shown in FIG. 11 measures illumination.
In an output curve in FIG. 11, the left side thereof indicates a high-illumination (bright) region, and the right side thereof indicates a low-illumination (dark) region. Here, a time period during which a voltage Vs of a fully charged capacitor decreases to a predetermined threshold value Vth is indicated by a time period t9 (bright state) and a time period t11 (dark state). As apparently shown in the output curve in FIG. 11, in the optical sensor formed by a TFT as an optical conductive element, since a current flowing by the optical leakage is minute in the low-illumination region, the period t11 until the voltage of the capacitor becomes the value not more than the threshold value becomes longer than the period t9 corresponding to the high-illumination region.
For this reason, an illumination detection period (sampling time) W is set to a predetermined long period of time value in order to use the optical sensor for the purpose of controlling the backlight unit of the liquid crystal display device, and particularly, for the purpose of the detection in the low-illumination region. In addition, a time period a, a process time period b, and a time period c are set to a predetermined period of time set in advance in accordance with a performance of a signal processor, where the time period a indicates a time period during which electric charge is fully charged in the capacitor, the process time period b indicates a time period during which a calculation is started and an illumination determination det is carried out, and the time period c indicates a time period during which the charging operation of the capacitor is started after the illumination determination det. Accordingly, since the W, a, b, and c are uniform, when a delay time period during which the voltage Vs of the fully charged capacitor decreases to the predetermined threshold value Vth and a calculation for the illumination determination is started is indicated by time periods t10 (bright state) and t12 (dark state),
                    W        =                ⁢                  a          +                      t            ⁢                                                  ⁢            9                    +                      t            ⁢                                                  ⁢            10                    +          b          +          c                                        =                ⁢                  a          +                      t            ⁢                                                  ⁢            11                    +                      t            ⁢                                                  ⁢            12                    +          b          +          c                                        =                ⁢        uniform            
When the above-described equation is rearranged,
                              W          -                      (                          a              +              b              +              c                        )                          =                ⁢                              t            ⁢                                                  ⁢            9                    +                      t            ⁢                                                  ⁢            10                                                  =                ⁢                              t            ⁢                                                  ⁢            11                    +                      t            ⁢                                                  ⁢            12                                                  =                ⁢        uniform            As is clear from above, the delay time period t10 in the bright state becomes longer than the delay time period t12 in the dark state. The delay time periods t10 and t12 are delay time periods until the illumination determination. For this reason, according to the known illumination measuring method, since the delay time period t10 in the bright state becomes longer than the delay time period t12 in the dark state, a problem arises in that the brightness control of the backlight unit is late particularly when the peripheral environment becomes bright suddenly.
Further, JP-A-2007-279100 discloses a technology in which a level of a gate voltage of an optical sensor formed by a TFT is changed for a low-illumination purpose and a high-illumination purpose in order to solve such a problem that a discharge time of the electric potential accumulated in a capacitor is long in the case where the external light is dark. However, it is disadvantageous in that a manufacture cost increases when a plurality of voltage application members is provided so as to apply a voltage to the gate voltage of the optical sensor formed by the TFT.